In recent years, lower power consumption of an electric instrument is required. According to the request, as a power-supply device that feeds power to circuits of respective units of the electric instrument, a switching power-supply device has been known which is configured to stop the power feeding to a circuit that is being not used at a standby state of the electric instrument and also to lower a voltage of the power that is fed to a circuit being used (for example, U.S. Pat. No. 8,054,653B2).
The switching power-supply device disclosed in the above Patent Document includes an AC/DC conversion unit, a switching element to which direct current power generated by the AC/DC conversion unit is input, a DC/AC conversion unit that generates a power-supply output of a desired voltage by a switching operation of the switching element, and a control unit that controls the switching operation of the switching element. The control unit detects a current flowing to a load and increases the voltage of the power-supply output at a heavy load where the current is relatively high and lowers the voltage of the power-supply output at a light load where the current is relatively low such as standby state of the instrument.